Injector sizing

[Greg Hermann]

At 9:04 AM 2/25/01, Tlsalt at aol.com wrote:
>Greg Hermann wrote:
>
>the .55 bsfc does it. Try looking at the HP AT the torque peak to figure
>out the pw (and duty cycle) you will need at the torque peak. HP is work
>per unit time, and injector flow rate is a part of the determination of
>fuel flow per unit time. They are consistent units. Torque has no time
>factor in it.
>
>Hello Greg:
>
>    I am still puzzled how a 20% increase in BSFC can account for a 50%
>increase in torque.

It doesn't. Brake SPECIFIC Fuel Consumption is fuel consumed per UNIT of
work done--a measure of efficiency, NOT a measure of fuel flow RATE  or of
the RATE of doing work (HP).

 If torque is proportional to the mass of air consumed
>doesn't the amount of fuel increase proportionally for a given air fuel
>ratio.

Yes, it does.

 Is there a thermal efficiency difference between NA and supercharged
>to explain it ?

Yep. A supercharged engine is significantly less efficient than an NA or a
turbocharged engine. Yes, a turbo motor is somewhat less efficient than an
NA engine (basically by the amount of compressor losses). BUT--a turbo
engine gets a significantly higher expansion ratio out of the working fluid
than either an NA or a supercharged engine does. This higher expansion
ratio is what brings its bsfc back closer to that of an NA engine. Note I
did not mention turbine losses. The expansion in the turbine is not quite
as efficient as the expansion in the cylinders, but it is close--that is
why I was silent on it.

 The pulsewidth at peak torque is higher than at peak
>horsepower, so I don't see how horsepower at peak torque is used to calculate
>peak torque pulsewidth.  Do you have a formula ?

It IS, but indirectly. The horsepower determines the DUTY CYCLE, not the
pulse width. Duty cycle = % of time open, not length of time open.
Pulsewidth is calculated from duty cycle TIMES maximum available cycle
time. At the torque peak--the duty cycle is lower than at the HP peak,
because the HP is lower, BUT the cycle time is enough longer so that the PW
ends up being longer at the torque peak than it is at the HP peak.

I used the VE based formula
>to calculate the mass of air at peak torque and then a 13:1 AFR to calculate
>the injector puslewidth as a base setting for the Haltech fuel curve. Since
>the puslewidth is relate to a single event and the amount of air inhaled per
>event is greatest at peak torque, pulsewidth at peak torque should be the
>longest in the fuel curve ? Do you agree ?

Absolutely.

  I understand the time bit and the
>maximum flow rate, duty cycle etc. for the ultimate fuel delivery in lbs/hr
>and the highest duty cycle I've seen in a data log was 84% at WOT at redline
>rpm.  I am having a hard time with this partially because the original torque
>curve of this engine was a peak at 1500 rpm and a flat line to 4500 rpm.

The flat torque curve should give a pretty constant PW from 1500 to 4500
rpm, but a linearly increasing duty cycle.

For example: 80 ms cycle time @ 1500 rpm with 20 ms PW and 25% duty cycle
AND  26.67 ms cycle time @ 4500 rpm with 20 ms PW and 75% duty cycle.
Constant torque from 1500 through 4500 rpm, and 3 times the HP (and  three
times the fuel flow) at 4500 rpm that there is at 1500 rpm. ( in these
numbers, BSFC is assumed to be CONSTANT from 1500 through 4500 rpm.)


  The
>problem my be entirely in my head.

Brain fade can be worse than brake fade! :-)

Greg


----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org